In an attempt to reduce the price of solar energy, many developments have been made with respect to lowering the cost of precisely repositioning and calibrating a surface with two degrees of freedom. In concentrated solar thermal systems, heliostat arrays utilize dual axis repositioning mechanisms to redirect sunlight to a central tower by making the normal vector of the heliostat mirror bisect the angle between the current sun position and the target. In order to properly align a heliostat's beam to a target, nine parameters must be defined. Three parameters are needed to define the heliostat's location relative to the receiver target. One parameter is needed to account for tolerances in pan and tilt home positions. One parameter is needed to define mirror-mounting offsets, and an additional parameter is needed to define the non-perpendicularity of the defined axes. Three final parameters are needed to define the heliostat's orientation in a global three-axis reference frame.
One method of defining these nine parameters is to use an over-constrained mathematical system. Precisely aligning a heliostat with this method requires a relatively large number of accurate samples that include information about the heliostat's geometric location and current pan/tilt angles relative to a known angle. The main problem with the current calibration approach is that in order to obtain an accurate and diverse set of samples, each heliostat must be calibrated relative to an accurately positioned sun or light-sensing device. For large heliostats (e.g., >20 m2) this may be accomplished with an attached sun sensor that tracks the sun throughout a day and compares known sun angles to angles measured by the heliostat's encoder system. Field workers must move this sun tracker from heliostat to heliostat until the calibration process is complete. For smaller heliostats this approach is not cost-effective as the reflecting area decreases while the amount of labor required per heliostat remains fixed. Micro-heliostat installers have attempted to solve this problem by placing sun sensors at known geometric locations in a field, and calibrating each heliostat to these sensors. This approach is problematic, as it requires precise installation of calibration towers/sensors and places constraints on heliostat installation flexibility that factor into the fully loaded system cost.
Similarly, calibration of photovoltaics (PVs) and concentrated photovoltaics (CPVs) trackers requires knowledge of the solar surface's orientation in a 3 axis global reference frame relative to a home pan and tilt position.